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Choose to view chapter section with a click on the section heading.

  • The Physics of Water
  • How Water Physics Affect Marine Life

Chapter Topic Menu

the physics of water
The Physics of Water

Seawater’s chemical properties affect how life functions in the oceans. Water’s physical properties not only affect life processes of marine organisms, but of human beings in the water.

Heat and Heat Capacity

  • Heat is the kinetic energy in the random movement, or vibration, of individual atoms and molecules in a substance.
    • The faster molecules move, the more heat there is. Total heat energy is measured based on both the quantity and speed of vibrating molecules.
  • Temperature measures only how fast the molecules vibrate.
    • The two most common temperature systems are Fahrenheit and Celsius. Celsius is most used in science because it is based on water’s physical properties.

The Physics of Water

Chapter 7 Pages 7-3 to 7-5

heat and heat capacity continued
Heat and Heat Capacity (continued)
  • Heat capacity of a substance is the amount of heatenergy required to raise a given amount of a substanceby a given temperature.
    • Scientists express heat capacity in terms of the amountof heat energy it takes to change one gram of asubstance by 1°C.
    • It’s expressed as the number of calories required.
    • It takes more heat energy to raise water’s temperature thanthat of most substances.
      • Therefore water can absorb or release alot of heat with little temperature change.
    • Water’s heat capacity affects the world’s climate and weather.
      • Heat is carried to areas that would otherwisebe cooler, and heat is absorbed in areas that would otherwise be hotter.

The Physics of Water

Chapter 7 Page 7-6

water temperature and density
Water Temperature and Density
  • As water cools it becomes denser. At 3.98°C (39.16°F) it reaches maximum density. Below this point, it crystallizes into ice. As water moves into a solid state* it becomes less dense.
  • Ice does not form all at once at the freezing point of 0°C (32°F), but crystallizes continuously until all liquid turns solid. Temperature does not drop any further until all the liquid water freezes, even though heat continues to leave.
    • This produces non-sensible heat – a change in heat energy that cannot be sensed with a thermometer.
    • The non-sensible heat lost when water goes from liquid to solid state is called the latent heat of fusion.
    • Sensible heat is that which you can sense with a thermometer.

* State is an expression of a substance’s form as it changes from solid, to liquid, to gas with the addition of heat.

Latent Heat of Vaporization

  • Latent heat of vaporization is the heat required to vaporize a substance.
    • It takes more latent heat to vaporize water than to freeze it because when water freezes only some of the hydrogen bonds break.
    • When it vaporizes, all the hydrogen bonds must break, which requires more energy.

The Physics of Water

Chapter 7 Pages 7-7 to 7-11

thermal inertia
Thermal Inertia
  • The tendency of water to resist temperature change is called thermal inertia.
  • Thermal equilibrium means water cools at about the same rate as it heats.
  • These concepts are important to lifeand Earth’s climate because:
    • Seawater acts as a global thermostat, preventing broad temperature swings.
      • Temperature changes wouldbe drastic between night and day and between summer and winter.
      • Without the thermal inertia,many – perhaps most – of theorganisms on Earth could not survive the drastic temperature changes that would occur each night.

The Physics of Water

Chapter 7 Pages 7-11 & 7-12

ocean water density
Ocean Water Density
  • Seawater density varies with salinity and temperature.
    • This causes seawater to stratify, orform layers.
  • Dense water is heavy and sinks below less dense layers. The three commonly found density layers are:
    • 1. Surface zone – varies in places from absent to 500 meters (1,640 feet). In general it extends from the top to about 100 meters (328 feet). This zone accounts for about only 2% of the ocean’s volume.
    • 2. Thermocline – separates the surface zone from the deep zone. It only needs a temperature or salinity difference to exist. This zone makes up about 18% of the ocean’s volume.
    • 3. Deep zone – lies below the thermocline. It is a very stable region of cold water beginning deeper than 1,000 meters (3,280 feet) in the middle latitudes, but is shallower in the polar regions. The deep zone makes up about 80% of the ocean’s volume.

The Physics of Water

Chapter 7 Pages 7-13 & 7-14

  • Water scatters and absorbs light. When light reaches thewater’s surface, some light penetrates, but, dependingon the sun’s angle, much may simply reflect back out ofthe water.
    • Within the water, light reflects off light-colored suspended particles.
    • Dark colored suspended particles and algae absorb some of the light.
    • Water molecules absorb the energy, converting light into heat.
    • Water absorbs colors at the red end of the spectrum more easilythan at the blue end.
  • Two zones exist with respect to light penetration:
    • 1. Photic Zone – where light reaches (can be as deep as 200 meters (656 feet). The photic zone has two subzones.
      • Euphotic Zone – the upper shallow portion where most biological production occurs – comprises about 1% of the oceans.
      • Dysphotic Zone – where light reaches, but not enough for photosynthetic life.
    • 2. Aphotic Zone – it makes up the vast majority of the oceans. Where light does not reach and only a fraction of marine organisms live.

How Water Physics Affect Marine Life

Chapter 7 Pages 7-16 to 7-20

  • Compared to land-based climates, marine organisms live in a much less challenging environmentwith respect to temperature range.
    • Ectotherm – An organism who's internal temperature changes with seawater temperature. Commonly called “cold-blooded.”
    • Endotherm – Organisms that have an internal temperature that varies, but remains 9°-16°C (48.2°- 60.8°F) warmer than the surrounding water.
    • Homeotherm – Have an internal temperature that is relatively stable. They are called “warm-blooded”; marine mammals and birds are in this category.
  • Temperature affects metabolism – the higher the temperature within an organism the more energy-releasing chemical processes (metabolism) happen.
    • Endotherms and homeotherms can tolerate a wide range of external temperatures.
  • Internal heat regulation allows endotherms an advantage.
    • Their metabolic rate remains the same regardless of external temperature allowing them to live in a variety of habitats.

How Water Physics Affect Marine Life

Chapter 7 Pages 7-21 & 7-22

  • Sound travels five times faster in water than in air.
    • It travels through warm water faster than cool…but it travels faster in deep water due to pressure.
    • Sound bounces off suspended particles, water layers,the bottom and other obstacles.
    • Sound travels much farther through water than light does.
    • Sound is eventually absorbed by water as heat.
  • Because sound travels so well in water, marinemammals use echolocation to sense an object’s size,distance, density, and position underwater.

How Water Physics Affect Marine Life

Chapter 7 Pages 7-22 to 7-24

  • Pressure exerted by water is called hydrostatic pressure.It’s simply the weight of the water.
    • At 10 meters (33 feet) hydrostatic pressure is equal to atmosphericpressure – 1 bar/ata.
    • At 10 meters (33 feet) the total pressure is 2 bar – 1 bar fromatmospheric pressure plus 1 bar from hydrostatic pressure.
    • A marine organism living at 10 meters (33 feet) experiencestwice the pressure present at sea level. Pressure increases1 bar for each additional 10 meters (33 feet).
  • Hydrostatic pressure doesn’t affect marine organismsbecause it is the same inside the organism as outside.
    • Living tissue is made primarily of water, which (within limits)transmits pressure evenly. Since it’s in balance, pressuredoesn’t crush or harm marine organisms.
    • Hydrostatic pressure is primarily an issue only for organismsthat have gas spaces in their bodies.

How Water Physics Affect Marine Life

Chapter 7 Pages 7-25 to 7-27

size and volume
Size and Volume
  • Using a sphere to substitute for a cell:
    • The volume of a sphere increases with the cube of its radius and the surface area increases with the square of its radius.
      • If a cell were to increase diameter 24 times original size, the volume would increase 64 times, but the surface areawould increase only 16 times.
    • High surface-to-volume ratio is important for cell function. The bigger the cell, the lower the surface-to-volume ratio, which means that there’s less relative area through which to exchangegases, nutrients, and waste.
      • This is why large organisms are multicellular rather than a giant single cell.


  • Archimedes’ Principle states that an object immersed ina gas or liquid is buoyed up by a force equal to the weightof the gas or liquid displaced.
    • This means marine organisms don’t have to expend much energyto offset their own weight compared to a land-based existence.
      • It allows entire communities to exist simply by drifting.
      • It allows organisms to grow larger than those on land.
      • It allows many swimming creatures to live without ever actually cominginto contact with the bottom.

How Water Physics Affect Marine Life

Chapter 7 Pages 7-28 to 7-31

movement and drag
Movement and Drag
  • Marine organisms avoid sinking by:
    • Plumes, hairs, ribbons, spines, and other protrusions that increase their drag and help themresist sinking.
    • Others have buoyancy adaptations that help them remain suspended in the water column.
  • Some marine organisms need to overcome drag as they swim. Adaptations thathelp them overcome drag:
    • Moving or swimming very slowly.
    • Excreting mucus or oil that actually lubricates them to “slip” through the water.
    • The most common is to have a shape that reduces drag – streamlining.


  • It is speculated that drifting provides several advantages.
    • 1. Drifting disperses organisms into new habitats, ensuring survival shouldsomething happen to the original community.
    • 2. May take organisms into nutrient-rich areas, preventing too manyoffspring from competing for the same resources in the original community.

How Water Physics Affect Marine Life

Chapter 7 Pages 7-31 & 7-32